The invention relates to a drive slippage control system for a four wheel drive vehicle, which system provides slippage values used for controlled braking of the wheels and/or controlling the drive torque. The slippage values are derived from the speed signals of the wheels and a reference value approximated to the vehicle speed derived from the wheel speed signals.
In case all-wheel driven vehicles start on low ground it is possible that all wheels are accelerated more rapidly as compared to the vehicle acceleration. For a high friction value, however, the wheel acceleration will conform with the vehicle acceleration. Based on this situation, it is difficult to form a reference value, which is required to determine the wheel slippage (drive slippage).